The present invention relates to a disk device that uses servo signals to conduct positioning control of a head, to a positioning control method for the head, and to a signal-processing circuit provided in the disk device in order to conduct a noise elimination process on the servo signals. More particularly, the invention relates to a disk device suitable for eliminating the impulse noise (and the like) superimposed on servo signals for reasons such as a disturbance, and to a positioning control method and signal-processing circuit for a head of the disk device.
The devices that use various forms of media, such as an optical disk or a magnetic recording tape, are known as data storage devices. Among them, a hard-disk drive (HDD) is most commonly used as a storage device in a computer, and is one of the storage devices indispensable in today's computer systems. In addition, the applications of the HDD, because of its excellent characteristics, are increasingly expanding not only in the computer field, but also as the removable memories (and the like) used in dynamic image recording/reproducing apparatus, car navigation systems, or digital cameras.
Such a data storage device, for example, a magnetic disk device that uses a head to read and write data, typically employs the so-called sector servo scheme. The sector servo scheme conducts positioning control of the head in accordance with servo data including a plurality of burst signals recorded in the servo regions of the servo sectors arranged on the disk.
Along with the enhancement of recording density in recent years, a magnetoresistive (MR) head utilizing an MR effect, a giant magnetoresistive (GMR) head utilizing a GMR effect, and an MR reading/thin-film recording composite head (hereinafter, referred to simply as the MR head) are coming to be used in magnetic disk devices. The MR head, as its head configuration exhibiting an excellent effect in the suppression of thermal asperity due to contact with a medium, records data using a thin-film head and reads the data by using a dual-stripe magnetoresistive (DSMR) head equipped with two MR elements.
By the way, in the MR head, if the magnetic sensitivity of the MR elements forming the MR head suddenly changes, this may cause so-called Barkhausen noise due to the superimposition of irregular noise on head read output. Noise due to an electrostatic discharge (ESD) caused by a disturbance may also occur, since the thin-film head, the MR elements, and the like are used in the head section. If such noise actually occurs, head position information that is to be originally derived from read-back signals of burst data will not correctly reflect the position of the head. Consequently, the positioning of the head will be impossible. With respect to this problem, for example, Japanese Patent Laid-open No. Hei 11-185203 (Patent Document 1) described below discloses a data read/write device that automatically detects the occurrence of irregular noise superimposition such as Barkhausen noise.
For the read/write device described in Patent Document 1, when positioning control of the head is to be conducted in a sector servo scheme using the burst signals A, B, C, and D contained in servo data, whether noise is superimposed on the burst signals is determined using burst outputs A to D that are the peak values of burst signals A to D. If noise is determined to be superimposed, write operation will be prohibited. The sum of the burst signals (i.e., A+B+C+D) is therefore used. More specifically, either the sum of the burst signals last determined to be free from the superimposition of irregular noise, or a predetermined expected value is held, the difference between the sum of the previous burst signals or the expected value and the sum of the current burst signals is calculated. If this difference is greater than a predetermined threshold value, write operation will be prohibited.
Also, even higher accuracy of head position detection is being demanded with the increases of BPI (Bits Per Inch) and improvement of TPI (Tracks Per Inch) in magnetic disks in recent years. Accordingly, instead of the so-called peak hold scheme where the position of the head is detected from such peak values of servo signals as mentioned above, the so-called area servo scheme where the waveform absolute values of servo signals are integrated and the head position is detected using the results of the integration, is used to conduct position control. The area servo scheme is described in, for example, Japanese Patent Laid-open No. Hei 10-255415 (Patent Document 2).
In this scheme, a servo detection circuit also detects the servo signals gain-adjusted by an automatic gain control (AGC) circuit provided at the preceding stage. Then, the servo detection circuit detects the current position of the magnetic head by use of burst signals contained in servo data, generates an error signal, and outputs the error signal to a controller. FIG. 10 is a block diagram showing the servo detection circuit described in Patent Document 2. As shown in FIG. 10, servo detection circuit 500 includes: a full-wave rectifier 501 for full-wave rectifying servo burst signals; an integrating circuit 502 for integrating the full-wave rectified servo burst signals; an A/D converter 503 for converting into digital data the integral value obtained by the integrating circuit 502; a zero-cross detector 504 for detecting the zero-cross points of gain-adjusted servo burst signals; and an integration control circuit 505 for counting the number of zero-cross points of servo burst signals received, and controlling the integrating circuit 502 so that when the count value reaches a previously set value, the integral value within the integrating circuit 502 will be held.
Servo signals are repeatedly recorded at predetermined periods, and the servo detection circuit 500 outputs, for example, 10 periods of burst signal integral data in accordance with the detection results obtained by the zero-cross detector 504. The controller at the following stage controls the positioning of the head by using, for example, the difference in integral value between bursts signal A and B. If noise is superimposed in the vicinity of the zero-cross point of a burst signal, it will not be possible for an accurate number of periods of integral data to be output, partly because the noise may be mis-counted as a zero-cross point. Therefore, according to Patent Document 2, not only zero-cross points are counted, but also is performed a process in which, during the time interval from the start of zero-cross point counting to the elapse of a predetermined time, integration is not stopped even if the count value exceeds the desired value.